(1) Field of the Invention
The invention relates to the general field of Liquid Crystal Displays, more particularly to the design of the black matrix.
(2) Description of the Prior Art
Referring to FIG. 1, the basic parts of a liquid crystal display are schematically illustrated in cross-section. A number of layers are involved, the outermost being a pair of crossed polarizers (not shown). In their most commonly used configuration, the polarizers are arranged so as to have their optic axes orthogonal to one another. That is, in the absence of anything else between them, light passing through the entrance polarizer would be blocked by the exit polarizer, and vice versa.
Immediately below the entrance polarizer is an upper transparent insulating substrate 2 (usually glass) and immediately above the exit polarizer is a similar lower substrate 1. Conducting lines (also not shown), running orthogonal to, and insulated from, one another are located on the lower surface of 2. Said orthogonal lines are connected at their intersections through Thin Film Transistors (TFTs). The TFTs allow voltage, separately applied to a set of orthogonal lines, to be added together only at the intersecting position which will overlie a given pixel of the display.
Sandwiched between, and confined there by means of suitable enclosing walls (not shown), is a layer of liquid crystal. Liquid crystals comprise long molecules, called nematics. The orientation of these molecules, relative to a given surface can be controlled by coating such a surface with a suitable orientation layer (not shown) and rubbing said orientation layer in the desired direction just prior to bringing it into contact with the liquid crystals.
Thus, in FIG. 1, the molecules closest to upper substrate 2 might be oriented so as to lie in the plane of the figure while the molecules closest to lower substrate 1 would be oriented to lie perpendicular to this plane. Molecules in between the two sets of pre-oriented molecules then arrange themselves so as to gradually change their orientations between these two extremes. Hence the term `twisted nematic` (TN) for such a configuration. A TN is optically active and will rotate the plane of any polarized light that traverses it.
Thus, polarized light that was formed and oriented as a result of passing through an entrance polarizer will be rotated though an angle of 90.degree. after traversing layer 9 and so will be correctly oriented to pass through the exit polarizer. Such a device is therefore normally on (transmits light).
An important property of TN is that, in the presence of an electric field (typically about 5,000 volts/cm.), normal to the the molecules, said molecules will all orient themselves so as to point in the same direction and the liquid crystal layer will cease to be optically active. As discussed above, a single pair of orthogonal lines comprise one electrode for generating said electric field, the other being transparent conducting common electrode 8, usually comprising indium-tin-oxide (ITO). Located between common electrode 8 and the pixels is overcoat layer 7.
To view a display of the type illustrated in FIG. 1, light may be applied from above the entrance polarizer, in direction 11, and then viewed from below the exit polarizer from direction 10, or a reflecting surface may be applied to the lower surface of the exit polarizer and the device viewed from above.
An important feature of LCDs is the black matrix, a cross-section of which has been designated 6 in FIG. 1. As can be seen, it is located at the spaces between sub-pixels 3. Its purpose is to block light that is extraneous to the display that would otherwise emerge on the viewing side of substrate 1, and thereby reduce the overall contrast.
Blockage of the undesired light by the black matrix occurs (in prior art) as a consequence of the reflection of incoming light, the conventional black matrix comprising a reflective metal layer such as chromium. While most of such reflected light never finds its way into the final image, some of it will, inevitably, get turned around, through scattering and through reflection at one or more of the several optical interfaces internal to the LCD, and end up contributing to the final image, thereby reducing its contrast level.
Attempts have been made in the prior art to reduce this unwanted reflection by the black matrix by coating it with a layer of light absorbing material such a polymer or an oxide, but this method is only partially effective, often allowing in excess of 3% of such light to dilute the final image.
Nishinski et al. (U.S. Pat. No. 5,307,189 April 1994) propose a solution to this unwanted light reflection problem wherein the black matrix, instead of comprising a metal such as chromium, comprises instead a photosensitive resin in which a black pigment has been dispersed. This choice of material has the advantage of effectively eliminating all reflection by the black matrix and, in principle, simplifying the overall manufacturing process since a separate layer of photoresist is no longer needed, the black matrix being formed directly from the resin layer after exposure and development.
Several disadvantages to this structure and method, as described by Nishinski et al, will now be considered:
Since a material comprising a photosensitive resin in which a black pigment has been dispersed will, by definition, severely limit the penetration of light, such a layer could have limited mechanical integrity. Furthermore, if the layer is inadvertently made too thick, the portion closest to the substrate will be under-exposed (or not exposed) and would be subject to unintended removal during development.
In general, polymer films do not adhere as well to glass substrates as do metallic films such as chromium.
A black matrix comprising only resinous material would not be electrically conductive. This is a disadvantage in applications where the black matrix is used to lower the series resistance of, for example, the common electrode.